1. Technical Field
This invention relates to loudspeaker horns, and in particular, to acoustic waveguides.
2. Related Art
Typically, loudspeaker horns consist of a driving unit that is coupled to a horn. The large end of the horn, called the “mouth,” typically has an area large enough to radiate sound efficiently at a desired low frequency. The small end of the horn, called the “throat,” has an area selected to match the acoustic impedance and exit diameter of the driving unit and to reduce distortion of the acoustic signal.
The loudspeaker horn is a directional control device that guides the acoustic signal or acoustic energy into particular directions or regions. The loudspeaker horn surfaces that constrain and control the radiation of acoustic energy are commonly referred to as an acoustic waveguide. The surfaces of an acoustic waveguide in a loudspeaker typically produce a coverage pattern of a specified total coverage angle that may differ horizontally and vertically. The coverage angle is a total angle in any plane of observation (although typically horizontal and vertical orthogonal planes are used). The coverage angle is evaluated as a function of frequency and is defined to be the angle at which the intensity of sound (Sound Pressure Level—SPL) is half of the SPL on the axis (the reference axial direction usually normal to the throat of the driver). Such a coverage pattern is desirable because it conforms to general audience seating patterns, for example seating patterns found in theaters, sports arenas and concert halls.
An acoustic waveguide defines a bounded region that directs the sound from the throat to the mouth of a horn with the throat being narrower than the mouth in both the primary horizontal and vertical planes. Acoustic energy radiates into the throat from the driver at high pressure, with a wave front that is nominally flat and free of curvature. As the wave front expands outward to the mouth of the horn or acoustic waveguide, the axial area increases in a uniform and monotonically increasing fashion controlled by an area expansion function designed to provide high acoustic impedance (or radiation resistance) at the throat.
The determined area expansion rate creates a uniform radiation impedance as a function of frequency to theoretically lower limit in frequency. The uniformly increasing area from the throat to the mouth yields a decreasing pressure gradient that couples the output of the driver to the free atmosphere. The free atmosphere provides low radiation impedance. This coupling of high acoustic impedance source to a low impedance load (the air surrounding the acoustic waveguide) provides an action analogous to an electrical transformer. The winding ratio is equivalent to the ratio of radiation resistance seen by the driver and the radiation resistance of the unrestricted surrounding atmosphere. In this analogy the drop in pressure from the throat to the mouth of the horn is equivalent to the voltage drop across a step down electrical transformer.
The shape of an acoustic waveguide affects the frequency response, polar pattern and the level of harmonic distortion of sound waves as they propagate away from the acoustic waveguide. As loudspeakers produce sound waves, waveguides are used to control the characteristics of the acoustic wave propagation. Current horn approaches include acoustic waveguide designs that have extruded curves that define the horizontal and vertical curvature in sheet surfaces. Other acoustic waveguide design approaches have designs that sweep the curvature about a point in space to create a quadratic surface (such as a hyperboloid.) In these examples, the intersection of the resulting four surfaces in a horn forms an interior surface that functions as the acoustic waveguide. The resulting acoustic waveguide has a circular entrance at the throat of the horn and an exit at the mouth. Current horn approaches often include a diffraction slot that is usually a rectangular slot, or a slice of a cylinder, in the throat and is defined as the narrowest width (height) of the horn surface to further control the characteristics of the acoustic wave propagation.
Constant directivity acoustic waveguide approaches have relied on curves that define the horizontal and the vertical curvature of the acoustic waveguide. The curvature in the horizontal and vertical planes fundamentally determines the frequency response (acoustic pressure as a function of frequency), as well as the acoustic polar pattern, or radiation pattern, of the acoustic waveguide and the level of harmonic distortion the acoustic waveguide creates due to the nonlinear behavior of the air at high pressures.
A constant coverage acoustic waveguide may be realized if the walls and expansion of the acoustic waveguide form a solution to the equations typically referred to as “Laplace's Wave Equation” by persons skilled in the art. Solving this equation for an acoustic waveguide of a desired coverage angle, and depth, and mouth dimension determines the correct area expansion rate in order for the wave front to remain perpendicular to, and attached to the sidewalls of the acoustic waveguide. As a result a diffraction slot is not required, and the compromises in performance are avoided.
However, the “solution” is limited to axis-symmetric acoustic waveguides that are simply surfaces of revolution formed by a single, two-dimensional curve, i.e. horns having round throat and round mouths, about the primary axis. As a result the coverage pattern of such devices is equal in both the horizontal and vertical planes, as well as at any intermediate plane between the two that pass through the primary axis.
Other approaches that realize approximate solutions for acoustic waveguides with coverage angles that differ in the horizontal and vertical planes, are typically formed from elliptical hyperbolas having three-dimensional surfaces that include a round throat and a closed elliptical mouth. The elliptical shape is used to approximate the solution to the wave equation because a circle is an ellipse with both the major and minor diameters equal to the diameter of the circle. From a practical standpoint this solution is non-ideal, as it does not use the available surface area of the mouth of the acoustic waveguide.
What is needed in the art is an acoustic waveguide that enables a wave front to expand smoothly and remain “attached” to the sidewall of the acoustic waveguide, without relying on geometric diffraction to produce constant directivity or constant coverage.